Field ion shadow projection microscope



March 31, 1970 E. w. MUELLER 3,504,175

FIELD ION SHADOW PROJECTION MICROSCOPE Filed Sept. 19, 1966 FIG. I FIG. 2

ION EMITTER "\BIoLoGIcAL 'L BIOLOGICAL I SPECIMEN SPECIMEN I I II II I I I FINAL I I I I SECOND ELECTRON l I I LENS IMAGE I I I l I I I; I I I I g gn I I I I INTERMEDIATE 6 I I I IoN IMAGE I I I I MAGNETIC ELECTRON FIELD M MIcRoscopE 52 mm mzzl \ zZa IMMERSION LENS 4 TARGET FINAL ELECTRON IMAGE INVENTOR.

ERWIN W. MUELLER Z JQQ BY ATTORNEYS.

United States Patent 3,504,175 FIELD ION SHADOW PROJECTION MICROSCOPE Erwin W. Mueller, State College, Pa., assignor to Research Corporation, New York, N.Y., a nonprofit corporation of New York Filed Sept. 19, 1966, Ser. No. 580,532 Int. Cl. H01j 37/26 US. Cl. 25049.5 5 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a microscope and more particularly to an ion microscope wherein ions emitted from a point ion source are employed to examine details of biological and other high molecular weight specimens.

It is well known in the optical art that the maximum resolution between two adjacent points on an object which is to be observed or examined is a function of the wave length of the light employed, either by way of transmission through or by way or reflection from the object to the lens system through which the observer looks. Accordingly there is a theoretical upper limit to the resolution obtainable with visible light, no matter how complex or refined a lens system may be. With the introduction and subsequent refinement of wave mechanics by theoretical physicists, workers in the microscope art were able to construct microscopes having much higher resolving powers than those using conventional light. Such microscopes are commonly termed electron microscopes and employ the wave properties of an electron. The wave length associated with an electron, accord ing to now well-known principles of wave mechanics, is smaller than the wave length of visible light and this recognition together with the construction of lenses capable of focusing electron beams (in a manner analogous to the focusing of a beam of visible light by a lens) resulted in the electron microscope. The electrons, as well as other charged particles, exhibit both a wave nature and a particle nature, this dualism made use of in such microscopes.

Ions are also susceptible of use in such microscopes, i.e., microscopes which utilize the much smaller wave length associated with a particle as opposed to the relatively longer wave length of visible light. Examples of such microscopes may be seen in US. Patents 2,548,870; 2,799,779 and 3,219,817. The use of ions for microscopy purposes is also found in the so called field-emission and the field-ion microscopes which utilize the emission of ions to image the surface of an approximately hemispherical tip of a fine needle. Rather high electric fields are required at the emitter surface and the final picture or image which is obtained is either an image of the emitter structure itself or the image of something which is very nearly an integral part or portion of the emitter, as for example a substance which is placed on the surface of the emitter. Such devices generally are used with specimens or emitters having rather high melting points and accordingly 3,504,175 Patented Mar. 31, 1970 are unsuitable for examination of specimens of high molecular weight which generally possess low melting points. In the case of a field emission microscope wherein electrons are employed the limit of resolution is approximately 25 Angstroms as distinguished from approximately three Angstroms when ions are used. Accordingly, the use of ions in a microscope employing the wave properties of particles will yield a superior limit of resolution by a factor of approximately 10.

According to the practice of the present invention, a field-ion emitter device, the details of construction of which are well known to workers in this art, is utilized to provide a light source having an effective diameter of approximately three Angstroms, i.e., an effective diameter of a single atom or of a single molecule. This ion light source is then projected through a biological or other specimen and subsequently impinges upon a target. Many of the ions passing through the specimen will not be deflected and will accordingly form an intermediate shadow ion image on a target. The target will emit electrons as a result of the ion bombardment, with the electrons being differentially emitted from those portions of the intermediate ion image which have been differentially bombarded by the ions. The electrons are now passed through an electron microscope of conventional construction for final imaging of the biological or other specimen. The first stage magnification attained by the ion beam bears the simple ratio of the distances between the specimen and the target and between the biological specimen and the ion emitter. Accordingly, the microscope of this invention may be termed a field ion shado w projection microscope.

In the drawings:

FIG. 1 is a schematic view showing a first embodiment of the field shadow projection microscope of this invention.

FIG. 2 is a view similar to FIG. 1 and illustrates a second embodiment of the invention.

Referring now to FIG. 1 of the drawings, the numeral 1 schematically designates a field ion emitter which serves as a light source. A single molecule adsorbed on the tip of the emitter or a single atom on the surface of the tip, designated by M, functions to emit ions of an externally supplied gas. The path of the ions from the tip of the field-ion emitter is indicated by the dashed lines. It will be understood that the effective diameter of the source of ions immediately below M is approximately 2.5 or 3 Angstrom units. Further details of construction'and of operation of the field-ion emitter 1 are omitted from this description for purposes of clarity, such details being well known to workers in this art.

The numeral 2 denotes a specimen holder for a biological specimen 3. The specimen may be a rather thin slice of biological tissue or the specimen may comprise any non-living matter having a rather large molecular weight. In their passage through the specimen in the indicated direction, some of the ions suffer deflection by the specimen while the unaffected ones impinge upon a target 4 at various points determined by the structure of the biological or other specimen 3. The target 4 may be termed an intermediate ion image in the sense that the ion image formed on the target 4 is intermediate because further magnification will be performed. As one example of the target 4, thin aluminum foil may be used although very nearly any metal foil may be used. As a result of the bombardment of the target 4 by the ions, so called secondary electrons are emitted from the target 4 and are focused and imaged in a conventional manner by an electron microscope 5. Generally, the target 4 will emit one or two electrons for each ion impinging upon it. It will be apparent that the target material is selected as one which will yield the maximum number of secondary electrons for each ion impinging thereon. Both the ion magnification stage as well as the electron magnification stage are suitably evacuated and positioned within a housing, as well known in this art.

To illustrate certain principles of the device, assume that the diameter of the ion source of the field-ion emitter 1 is 3 Angstroms. It will be recalled that this figure represents the size of a molecule or an atom on the very tip of the pointed field ion emitter 1. If the distance between the source M and the specimen 3 is one-tenth of a millimeter, and the distance between the biological specimen 3 and the target 4 is 100 millimeters, then the so called primary magnification will be one-thousand, this representing the ratio of these two distances. It will now be recalled that the limit of resolution of the human eye is approximately 10- centimeter. If the magnification of the electron microscope 5 is made such that it will magnify the product of the resolution and of the magnification of the primary ion shadow projection portion of the device (the product in the above example being 3X10" cm.) then a human eye at the final electron image plane will enjoy maximum etfectiveness of the small diameter of the ion emitter source. In the example given, the electron microscope 5 would be required to yield a magnification of only /a' 1O this being the magnification required to bring the three Angstrom light source size as represented by the ion or atom at M from the already magnified shadow projection technique to the centimeter limit of the resolution of the human eye. Still further magnification by the electron microscope 5 would serve no useful purpose except to increase fuzziness since the ultimate limit of resolution is determined by the three Angstrom (in the example given) size of the source, M. The particular numerical values employed in the example above are by way of illustration only. Further, as well known to workers in this art, the final electron image may be captured by a photographic film at the indicated final image plane in which case the magnification required by the electron microscope 5- need not be as great as in the example given wherein the human eye is employed directly.

Turning now to FIG. 2 of the drawings, an embodiment of the invention is illustrated and corresponding elements have been designated with the same numerals. The general mode of operation is the same with the exception of the precise manner of the formation of the interme' diate ion image and the subsequent formation of the final electron image, the cooperation between the field ion emitter 1 and the biologocal specimen 3 being the same as in the embodiment of FIG. 1. The target 4 in the embodiment of FIG. 2 is a reflection or backscattering target as distinguished from the transmission target 4 of the embodiment of FIG. 1. This means that the target 4 in the embodiment of FIG. 2 may be much thicker than the foil target of the previous embodiment and the practical result or difference in operation is that with the thick target of the embodiment of FIG. 2 a greater number of secondary electrons are emitted for every bombarding ion. In this second embodiment, the velocity of the ions which pass through the biological or organic specimen 3 and impinge upon the target 4 is greater than in the previous embodiment and this greater velocity causes the above noted difference in the number of secondary electrons for each incoming or bombarding ion, this difierence may be four or five secondary electrons for each bombarding ion, as distinguished. from one or two in the case of the thin or transmission target 4. The higher ion velocities of the embodiment of FIG. 2 would be impractical with the embodiment of FIG. 1 because the foil or other thin metal 4 would have an extremely short life with the higher ion velocities.

Continuing with the description of the embodiment of FIG. 2, a magnetic field designated by the numeral 6 is maintained at right angles to the plane of the drawings by any conventional means. After their emission from the target 4, the so called secondary electrons induced by the bombardment of the incoming beam are focused by the immersion lens 5 whose function is to provide the first stage magnification of the electron microscope section with the electrons now passing through the field 6 and undergoing a deflection of the order of magnitude of as illustrated. After the deflection, the secondary electrons pass through the lens 7, a conventional electrostatic lens used in electron optics, for a final imaging at the indicated plane.

As illustrated at FIG. 2 of the drawings, the incoming ion beam is only slightly deflected by the field 6 while the secondary electrons are greatly deflected by this field. This will illustrate the relative stability of an ion beam as opposed to an electron beam in its lesser response to external magnetic fields. With the greater number of secondary electrons emitted for every bombarding ion, it will be apparent that the embodiment of FIG. 2 of the invention yields greater sharpness and detail even though it is more complex than the embodiment of FIG. 1.

According to the practice of this invention, by virtue of the ion shadow projection technique employed it is possible to obtain an ultimate resolution of the order of 2 to 3 Angstrom units combined with the excellent contrast conditions provided by the use of ions instead of electrons. This is of paramount importance for the imaging of biological and organic molecules. A few present electron microscopes have reached a limit of resolution of the order of 3 Angstroms. However, such resolutions have been obtained in showing net planes of metal crystals and such microscopes perform extremely poorly with specimens of low atomic weight such as biological molecules. Here, in the case of biological molecules, a resolution of 30 Angstroms is rarely approached because of the poor contrast conditions.

At the initiation of operation of the microscope, the position of the tip of 1 may have to be varied slightly with respect to the specimen 3 to insure that there is an adsorbed ionizing molecule M of the externally supplied gas, or an ionizing atom on the tip, which will generate the indicated ion path. In practice, the user may make this adjustment until the expected resolution is realized.

The microscope of thi invention exhibits great simplicity in the first or ionic magnification stage. No focusing lenses are required for the ion beam, as distinguished from prior ion stream devices such as illustrated in US. Patent 2,799,779.

The practice of this invention is not entirely limited to the use of an electron microscope as the second stage, i.e., a stage for imaging the effect of the first stage. Thus, the ion target may be a conventional grainless (evaporated or vapor deposited) phosphorescent screen, with the ions causing a glow thereof viewable by a high power conventional optical microscope. Alternatively, the low intensity of the ion image on such a screen may be amplified by conventional photoelectronic image intensifiers.

What is claimed is:

1. A field ion shadow projection microscope comprising a field ion emitter having an effective diameter in the order of approximately 2.5 Angstroms and an externally supplied gas for eifecting the emission of a divergent beam of ions, target means in the path of said ion beam effective to convert the ions in said beam into electrons, means positioning a specimen in the beam of ions intermediate the emitter and the target means and field means forming an image of the electrons emanating from the target.

2. A field ion shodow projection microscope as defined in claim 1 wherein the image-forming field means act upon ions emanating from the target in the same direction as the direction of the ion beam impinging on the target.

3,504,175 5 6 3. A field ion shadow projection microscope as de- References Cited fined in claim 1 wherein the image-forming field means UNITED STATES PATENTS act upon ions emanating from the target in the direction counter to the direction of the ion beam impringing on 2,548,870 4/1951 f 'l th target 2,799,779 7/ 1957 Werssenberg.

4. A field ion shadow projection microscope as defined 5 3,277,297 10/1966 et in claim 1 wherein the field means include electri field 3,336,475 8/1967 patrlck 25043 forming means.

5. A field ion shadow projection microscope as defined RALPH NILSON Primary Examiner in claim 1 wherein the field means include magnetic field 10 A. L. BIRCH, Assistant Examiner forming means. 

